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But I've really saw a photo in "Krokodil" magazin in the end of 80's and there was "Береги природу...."[/quote]
I haven't seen any signs like that until now. :) I only knew it as a play on words.

Another version is "Люби Родину - мать твою!"

In Russia we have a sign "Осторожно дети" which is funny because it can be interpretated as "beware of children". But there's a certain share of truth in that. :)